This invention relates generally to printed image inspection apparatus, and more particularly to printed image inspection apparatus that reads as an image the image produced by merging images formed by a plurality of color inks and compares the merged image with reference images produced by the same number of color inks. More specifically, this invention relates to printed image inspection apparatus for inspecting a printed image being inspected by applying two types of light having different dominant wavelengths onto the printed image being inspected, reading images from the reflected light, and comparing them with reference images stored in advance.
A printed image inspection apparatus for inspecting a printed image being inspected by applying two types of light having different dominant wavelengths onto the printed image being inspected, reading images from the reflected light, and comparing them with stored reference images is well known. The printed image inspection apparatus of this type is based on the fact that inspection of full-color printed matter does not necessarily requires the separation of color images, unlike the reproduction of a color print, as disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080, for example.
The printed image inspection apparatus disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080 relies on the fact that blue light is absorbed better by yellow or magenta (a reddish purple) ink than by cyan (a bluish green) ink, while red light is absorbed better by cyan ink than by yellow or magenta ink.
An image is read when light of blue and red colors is applied on a printed image being inspected, and the reflected light is received by a light receptor. The image read by applying blue light is compared with an image obtained by merging the reference images of the yellow, magenta and black inks of the printed image being inspected that has been stored in advance. The image read by applying the red light is then compared with the image obtained by merging the reference images of the cyan and black inks of the printed image being inspected that has been stored in advance. In this way, the printed image being inspected is inspected through these comparison.
In studying printed image inspection apparatus for inspecting full-color printed matter without splitting the printed image into images of various colors, we gave consideration to the following points.
Firstly, yellow, cyan, magenta and black inks are usually used in full-color printing. Spectral distribution curves (curves indicating the reflectance of light of each wavelength) of yellow, cyan and magenta inks are as shown in FIGS. 13, 14 and 15. These curves are described in page 153 of xe2x80x9cIro no Ohanashixe2x80x9d (A Story on Colors) (Japanese Standard Association, Nov. 20, 1992). Black ink has a very low reflectance close to zero for light of any wavelength.
Secondly, JIS (Japanese Industrial Standard) P3101 (printing paper) provides that the brightness of uncolored printing paper be at least 55%. The brightness of printing paper is the value obtained in accordance with JIS P8123 (Hunter brightness testing methods for paper and pulp) that is almost equal to the reflectance of the paper when the paper surface is irradiated with the light of a dominant wavelength of 457.0xc2x10.5 nm (blue light). JIS P3001 (newsprint paper) allows newsprint to have a brightness of 45%, min., almost equal to that of printing paper.
Thirdly, the printed image inspection apparatus disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080 applies light to a printed image being inspected, and reads the image by the reflected light. In this case, the amount of reflected light is lessened compared with the amount of reflected light from the paper surface because the printed image being inspected absorbs part of the applied light. The printed image inspection apparatus disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080 reads by a light receptor the difference in the amount of reflected light as contrast. The higher the degree of contrast, that is, the print contrast signal value (hereinafter referred to as the PCS value for short), the more accurately the apparatus read the image. The PCS value is calculated by the following equation.
PCS=(reflectance of paper surfacexe2x88x92reflectance of image)/reflectance of paper surface
In general, any optically readable image has a PCS value of more than 0.5; a practically readable image has a PCS value of more than 0.6, or preferably more than 0.8. This is described in pp. 215 to 216 of xe2x80x9cShikisaigaku Nyumonxe2x80x9d (Introduction to Chromatics) (Nihon Insatsu Shimbunsha, Nov. 15, 1988), for example.
Furthermore, based on the first through three points, it can be considered that practically effective images can be read by irradiating the paper being inspected with blue light similar to that for measuring the brightness of paper, as mentioned earlier (light of a dominant wavelength of 430 to 490 nm), for example. In this case, the reflectance of the image being read must be a value that can make the PCS value more than 0.6, where the PCS value is determined by the relationship between the image and the brightness (the value almost equal to the reflectance of the paper with respect to blue light) of the paper on which the image is printed. In addition, it is considered that the brightness with respect to blue light (light having a dominant wavelength of 457.0xc2x10.5 nm) of printing paper as specified by JIS is 55%, min., and the brightness with respect to blue light of newsprint as specified by JIS is 45%, min. From the above discussion, it is found that the blue-light reflectance of images printed on these paper types must be less than 18% in order to read images printed on printing paper and newsprint as specified by JIS through irradiation of blue light to a practically effective degree.
Based on these considerations, we studied the stain detection performance of printed image inspection apparatus that inspects full-color printed matter without splitting the image into images of each color, and obtained the following observations.
The printed image inspection apparatus disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080, for example, reads a printed image as the image obtained by merging yellow-ink, magenta-ink and black-ink images using blue light, as described above, and compares the read image with the image obtained by merging the reference images of yellow-ink, magenta-ink and black-ink. That is, the printed image inspection apparatus cannot separate yellow from magenta. The printed image inspection apparatus that cannot separate yellow from magenta cannot detect magenta-colored stains on a yellow background, and yellow-colored stains on a magenta background.
As is apparent from FIG. 16 combining spectral distribution curves shown in FIGS. 13 through 15, there is no region where both the reflectance values of yellow and magenta inks with respect to blue light (light having a dominant wavelength from 430 to 490 nm) are less than 18%. Two wavelengths (470 and 590 nm) shown in FIG. 16 are those of the light sources selected for the present invention. The printed image inspection apparatus disclosed in Japanese Published Unexamined Patent Publication No. Hei-9(1997)-156080 can inspect full-color printed images on printing paper and newsprint, both having high brightness values, except for magenta-colored stains on a yellow-colored background and yellow-colored stains on a magenta-colored background, both described earlier. With the printed image inspection apparatus using blue and red light, however, either of yellow-ink or magenta-ink images cannot be read on printing paper or newsprint having low brightness values even if the printing paper or newsprint satisfies JIS standards in terms of brightness. For this reason, printed image inspection apparatus using light having no range where both the reflectance values of yellow and magenta inks are less than 18% may be incapable of inspecting printed images containing images of these inks.
It is an object of the present invention to provide printed image inspection apparatus that can detect not only yellow, cyan and magenta stains on a white background, but also cyan and magenta stains on a yellow background, yellow and magenta stains on a cyan background, and yellow and cyan stains on a magenta background.
It is another object of the present invention to provide printed image inspection apparatus that can inspect with high accuracy full-color images on printing paper and newsprint, both satisfying at least JIS standards in terms of brightness, using two types of light.
It is a further object of the present invention to provide printed image inspection apparatus that can inspect with relatively high accuracy full-color images on printing paper and newsprint, both having low brightness values, using two types of light.
The printed image inspection apparatus of the present invention comprises an image reading section for irradiating a printed image being inspected with two types of light having different dominant wavelengths and reading the image from the reflected light, a judgment section for comparing the image read by the image reading section with the reference image that has been stored in advance to judge agreement between both, and an operation control section for controlling the operations of the image reading section and the judgment section; the image reading section having first and second light sources, with the first light source being one of the light sources of the aforementioned two types of light outputting blue light having low reflectance values with respect to yellow, magenta and black inks, and the second light source being the other light source outputting yellow light having low reflectance values with respect to cyan, magenta and black inks.
According to an embodiment of the present invention, the first light source emits blue light having a dominant wavelength in a range of 430-480 nm, and the second light source emits yellow light having a dominant wavelength in a range of 540-590 nm.
According to another embodiment of the present invention, the first light source emits blue light having a dominant wavelength of 470 nm, and the second light source emits yellow light having a dominant wavelength of 590 nm.
According to the printed image inspection apparatus of the present invention, yellow and magenta stains on a white background can be detected by blue light, and cyan and magenta stains on a white background can be detected by yellow light. Magenta and cyan stains on a yellow background can be detected by yellow light, and yellow and magenta stains on a cyan background can be detected by blue light. In addition, a yellow stain on a magenta background can be detected as a lack of the magenta background by yellow light, and a cyan stain on a magenta background can be detected as a lack of the magenta background by blue light.
Inspection of a printed image on paper having low brightness inevitably involves a slight drop of the detection accuracy of magenta stains on a cyan background and cyan stains on a magenta background. Furthermore, inspection of a printed image on paper having low brightness inevitably leads to uncertainty in reading magenta-ink images. These drawbacks, however, hardly pose practical problems because there are few cases of full-color printing on paper having low brightness.